1. Field of Invention
The present invention relates to processes and apparatus for removal of unwanted substances from thermally produced gases and, more specifically, to the removal of acid gases, hydrogen chloride and hydrogen sulfide gases, and toxic metal vapors such as mercury, and lead, from the same.
2. Description of Prior Art
Renewable opportunity fuels such as lignocellulosic biomass (“biomass”) and refuse derived fuels (“RDF”) from municipal or industrial waste are important feedstocks for future production of renewable power and synthetic fuels and chemicals. However, some of these fuels (especially rapid growing biomass and RDF) contains chlorine and other contaminants. Chlorine may exist in the ash as an inorganic salt, or may be bound to carbon (in organic form). The combustion or gasification of biomass (including RDF) will contribute to the release of hydrogen chloride (HCl) gas, which is a hazardous air pollutant (HAP) if emitted. Use of refuse derived fuels can also produce vapors of toxic metals if present in the waste feedstock, for example, mercury and lead.
There are also several natural sorbent elements present in biomass ash (including common alkali metals, potassium, and sodium; also common alkali earth metals, calcium, and magnesium; and transition metal oxides, titanium dioxide, zinc oxide) that have affinity for acid gases at certain temperatures—usually less than the gasifier operating temperature—and also for toxic metal capture. Indeed, there are various natural minerals such as dolomite and calcite (limestone) that have rapid kinetics for hydrogen chloride and hydrogen sulfide gas capture if activated—usually by heating—that can be employed, along with the natural biomass ash, to reduce acid gas concentrations in generated gases prior to combustion. Further, studies show the contaminant is more concentrated in the generated gas and therefore if captured to low concentrations (limited by equilibrium and sorption kinetics) in the smaller volume generated gas, then the resulting emission would be lower in net (after combustion) than by post combustion flue gas cleaning alone.
An example acid gas (HCl) capture scheme using calcite (limestone) is as follows:
                            ⁢                                            CaCO              3                        ⁡                          (              s              )                                ⁢                      ↔            heat                    ⁢                                    CaO              ⁡                              (                s                )                                      +                          CO              2                                                                  ⁢                  (                      Calcite            ⁢                                                  ⁢            to            ⁢                                                  ⁢            activated            ⁢                                                  ⁢            lime                    )                                                              CaO            ⁡                          (              s              )                                +          HCl                ↔                  CaOHCl          ⁡                      (            s            )                                              (                  First          ⁢                                          ⁢          capture                )                                        ⁢                                            CaOHCl              ⁡                              (                s                )                                      +            HCl                    ↔                                                    CaCl                2                            ⁡                              (                s                )                                      +                                          H                2                            ⁢              O                                                                  ⁢                  (                      Second            ⁢                                                  ⁢            capture                    )                    
Overall capture reaction (from oxide phase):CaO(s)+2HClCaCl2(s)+H2O (Net capture, lime to calcium chloride)The reaction rate for hydrogen chloride capture by calcium oxide (CaO) is reported to be first order with respect to HCl (Li, M, Shaw, H, and Yang, C. L., “Reaction Kinetics of Hydrogen Chloride with Calcium Oxide by Fourier Transform Infrared Spectroscopy.” Ind. Eng. Chem. Res. (39), 2000: 1898-1902), and rate limited by surface reaction—provided internal mass transfer resistances are negligible (small particles, small grains) and an excess surface area is available. Kinetics for this reaction are reported in the literature (Shemwell, et. al. 2001, Gullet, et. al 1992, Li, et. al, 2000).
Published approaches for removal of chlorides and acid gas include introducing gas to be treated into a non-pressurized (atmospheric pressure) circulating fluidized bed of limestone—a gas treatment device—that contacts the treated gas with a high excess of sorbent, usually in a post combustion flue gas stream. This method operates at relatively lower temperatures and so requires large contact volumes. When operating in post combustion systems, the gas must be reheated to have effective kinetic performance. (FGD TECHNOLOGY DEVELOPMENTS IN EUROPE AND NORTH AMERICA, Wolfgang Schuettenhelm, Thomas Robinson, and Anthony Licata, © Babcock Borsig Power, Inc., 2001.)
Also known is the injection of prepared ultra-fine activated powders (dry or wet) into a capture system—which could be the freeboard of either a fluid bed combustor or fluid bed gasifier. Injection of powdered non-activated limestone, dolomite, or slaked lime into the produced gas or flue gas has also been used. However, injecting cold non-activated powders requires additional time for the powder to be warmed to the gas temperature. These other disclosures do not achieve the quality of particle dispersion that might increase the efficiency of the process. (U.S. Pat. No. 5,464,597)
Limestone and dolomite are commonly used as sorbents in atmospheric circulating fluidized bed gasifiers. The circulating fluid bed relies on the sand recovery cyclone efficiency to influence particle slip; therefore, because the cyclone has a fixed geometry it cannot modulate with gas production capacity to effect any benefit for controlling sorbent and bio-char particle size with gas production capacity. Moreover, the atmospheric system cannot modulate any parameter with capacity to maintain ideal superficial velocity for sorbent particle size quality, nor maintain downstream residence time as constant. (Combustion and Gasification in Fluidized Beds, Prabir Basu, (2006, CRC Press, Taylor and Francis Group)
A common approach to the problem of removing unwanted substances such as chloride and acid gas is to inject finely divided (<40 μm) dry powder sorbents into the target gas stream (Shemwell 2001); alternately, a sorbent slurry might be sprayed into the gas with the sorbent in hydrated form (e.g., slaked lime, Ca(OH)2(s)). The kinetics of chloride capture (for example) is benefited by having smaller particles to reduce internal diffusion limitations and improve sorbent utilization.
What is needed is a system that can achieve the removal of chlorides and acid gas while operating in a smaller volume gas stream at higher temperatures with less kinetic limitations. Further, a system that does not require gas reheating would be beneficial. Finally, a system that could be operated to produce particles of sorbent of desirable size and modulate parameters in order to maintain ideal superficial velocity for sorbent particle size quality and to maintain constant downstream residence is highly desirable.